Viruses May Cause More Cancer than Previously Thought

This image shows a single cell grown from a culture of lung epithelial cancer cells. The purple spheres are 'blebs': irregular bulges where the cell's internal scaffolding - its cytoskeleton - becomes unlinked from the surface membrane.
This image received an award from the Wellcome Trust, as part of the annual Wellcome Image Awards, for its ability to communicate the wonder and fascination of science.Credit:

Viruses may be causing more cancers than previously thought,
according to a new study.

Scientists know that a few cancers, such as cervical
cancer, are caused by viruses,
because researchers have isolated the viral genomes from the cancerous cells.
But some viruses may take a "hit and run" approach — inducing cancer
and then vanishing before the disease is caught, the researchers say.

The new study, performed in mice, showed that a particular
mouse herpesvirus could trigger cancer but then practically disappear from the
cancer cells. Herpesviruses belong to a family of viruses called Herpesviridae that can infect humans and include chicken pox and the Epstein-Barr virus — a virus that virtually everyone is infected with, yet only causes cancer in rare cases.

"We're saying that herpes viruses could cause a lot
more disease than you would guess if you limited your suspicion to just to
cancers that came out having the virus genome in them," said study author
Philip Stevenson, a researcher at the University of Cambridge in the United
Kingdom. "We're saying that they might cause orders of magnitude more
cancers," he told LiveScience.

However, it is important to note that the cancers induced in
the mice are different from the cancers that occur in people, and so the findings
might not hold true for humans.

The results are published in the September issue of the Journal of General Virology.

Hit and run

Cancers occur when the genetic material within cells, the cells' DNA, develops
mutations that cause the cell to divide uncontrollably. These mutations
sometimes arise when the DNA is damaged. However, viruses can have effects
similar to these mutations when they insert themselves into the DNA, and the
end result is the same – uncontrolled cell growth. Two types of human
papillomaviruses (HPV) are thought to cause cervical cancer this way.

By inserting itself into the cells' DNA, the virus
"hides" from the body's immune system, so the body's defenses can't
eliminate it. This means that researchers can later find evidence that HPV was
indeed the culprit behind cervical cancer — the virus leaves its genes behind
like fingerprints at a crime scene. About 20 percent of cancers are thought to
be caused by viruses, according to Stevenson.

However, cells have several defense mechanisms to prevent
tumor growth. Sometimes cells with certain mutations will self-destruct so they
don’t turn cancerous.

But viruses can interfere with these defenses.

"Viruses don't set out to cause cancer, but their
replication uses all the same functions. So they tend to inhibit the whole set
of these protective mechanisms," Stevenson said. "So they're kind of
ideal agents for causing cancer."

The "hit and run" hypotheses proposes that a virus
can cause cancer without integrating itself into the cell's DNA. In this case,
a cell develops a genetic mutation, but the virus present in the cell overrides
the defense mechanisms and allows the cell to continue to live. Over time, more
and more genetic mutations develop, and the cell turns cancerous. However, by
the time the cancer is discovered, the virus has been eliminated by the immune
system, leaving no "fingerprints" behind.

For many years scientists have suspected viruses could cause
cancer through this mechanism, but it was difficult to prove.

Cancer vaccines

Stevenson and his colleagues wanted to know: if a virus
triggers cancer without integrating into the cells' DNA, how often can you
still find traces of the virus inside the cancer cells?

They genetically engineered mice so that they would develop
cancer if infected with the herpesvirus (the virus has a gene that triggers mouse
cells to divide uncontrollably).

But when they later checked the tumors that grew in the mice,
they found no trace of the virus. If the findings hold true for humans, that
would mean viruses such as Epstein–Barr trigger more cancers than previously thought.

Further, the researchers found evidence that vaccinating
against these viruses might prove effective in preventing cancers. When the
researchers vaccinated the mice against the herpesvirus, none of the mice
developed cancer.

While a vaccine
against Epstein–Barr and other herpes viruses could theoretically be made,
companies would not be willing to take the risk to develop them unless there
was some significant disease threat, Stevenson said.

"With every vaccine there's a balance between the risk
and the benefits," Stevenson said. Because cancers that have the Epstein-Barr
virus genome in them are thought to be relatively rare, the benefits are seen
as small, so no one wants to take the risk, he said. "If people recognize
that these viruses may cause an awful lot more cancers, then the benefits [become
greater]."

The current study suggests that a clinical trial of an
Epstein–Barr vaccine or similar vaccine might be worthwhile, Stevenson said.

Rachael Rettner

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.